Northern Virginia Winter Weather

Barry A. Klinger, Winter 2014, 2015
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Sections:
Cold Winters, Snowy Winters, and Global Warming
Are DC Winters Getting Colder?
120+ Years of Snowfall

Cold Winters, Snowy Winters, and Global Warming

[PDF version of figure].

Washington DC has had some bitter winters including "Snowmaggedon" in 2010 and the so-called "Polar Vortex" rampaging south across the DC metropolitan area and much of the US in 2014. The figure at top left shows Washington DC monthly average temperature anomalies. It shows some of the cold events along with warm events such as March 2012. Do the cold spells and snow storms show that global warming is not happening?

Local temperatures have much more dramatic swings than global temperature. The bottom left panel shows the global average temperature anomaly. This has much smaller ups and downs and does not even go in the same direction as the Washington temperature. Even the average temperature over the 5 degree wide "square" centered at (77.5oN,39.5oW) shows less variation than temperature measured at the airport (top left panel, dotted black line). However, we can see that recent global temperatures (blue and red bars) are all much warmer than temperatures from the early 1970s (light blue bars - 1970 temperatures are shown in 2010, 1971 in 2011, etc.). Global warming has occurred over recent decades.

Why is local and global temperature so different? It is because weather moves heat around from location to location, cooling some regions and warming others. The global temperature maps (right panels) shows this effect. When the eastern US was cold (top two maps), there were compensating warm regions in Canada, Africa and Asia.

By the same token, hot weather in DC (March 2012, bottom right panel) is also partly compensated by cool regions in Alaska, Australia, and Siberia. Local weather can not prove or disprove global warming. However, global warming implies that as time goes on, there will be more warm months and fewer cool months.

Some details about maps:Global temperature maps are from the NOAA Merged Land-Ocean Surface Temperature Analysis (MLOST) downloaded March 2015. Maps are equal-area so that two regions that have equal area in the figure represent regions that have equal area on the globe. Note that for stronger temperature anomalies, the contour interval is also bigger. This allows more precision for large areas with weak anomalies while preventing clutter from too many contours in small areas with strong anomalies.

Jason Samenow (Capital Weather Gang) on the cold February and global warming

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Are DC Winters Getting Colder?

A recent article by the Washington Post Capital Weather Gang's Matt Rogers highlights a cooling trend in "colder-trending" extreme temperatures during Washington-area winters since 2000. The article was based on statistics of high and low temperature extremes during recent winters.

A more straightforward way to examine the seasonal temperatures is to simply look at averages rather than extremes. The plot above shows monthly average temperatures (red circles) for each December, January, and February since December 2000. The graph also shows seasonal averages (December to February). Neither measure of winter temperature shows any obvious trend. Fitting a straight line to the trend produces a slight warming, but it is not statistically significant (as measured by a correlation coefficient of r2=.01). As an example of how fragile the small warming trend is, note that changing the year 2001 temperature from 36 to 40 would be enough to change the warming trend to a cooling trend.

For a more complete discussion of trends in DC-area temperatures, see the next section.

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120+ Years of Snowfall

DC-area annual snowfall has always had tremendous variations. As the top panel of the figure shows, alternating years can have snowfalls ranging from 10 inches to 40 inches.

Despite the record breaking snowfall in 2009-2010, the long-term trend in snow is actually downward. The 19-year running mean shows this in the middle panel of the figure. Ian Livingston of theWashington Post Capital Weather Gang has an interesting discussion about the decreasing snowfall. He shows that the drop-off has been mostly at the beginning and end of winter. Those months have fewer cold days than they used to and hence many weather events that would have caused snow a few decades ago are now giving us rain.

The bottom panel in the figure shows how many years had each range of snowfall. It shows that the most common amount was 20-30 cm (8-12 inches), with most years getting no more than 80 cm (31 inches). About 10% (very roughly) of the years since 1880 have had more than a meter of snow.

Local geography of snowfall

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Last modified: 20 February 2019